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Wednesday, July 04, 2007

What's an advanced algorithm ?

And he's back !!!

After a rather refreshing (if overly hot) vacation in India, I'm back in town, forced to face reality, an impending move, and the beginning of semester. Yep, the summer is drawing to a close, and I haven't even stuffed my face with hot dogs yet, let alone 59.5 of them (is there something slightly deranged about ESPN covering "competitive eating" as a sport?).

Anyhow, I digress. The burning question of the day is this:

What is an advanced algorithm ?

It might not be your burning question of the day, but it certainly is mine, because I am the proud teacher of the graduate algorithms class this fall.

Some explanation is in order. At most schools, undergrads in CS take some kind of algorithms class, which for the sake of convenience we'll call the CLRS class (which is not to say that it can't be a KT class, or a DPV class, or a GT class, or....). At any rate, this is the first substantial algorithms class that the students take, and often it's the last one.

Now when you get into grad school, you often have to complete some kind of algorithms breadth requirement. In some schools, you can do this via an exam, and in others, you have to take a class. Namely, a graduate algorithms class. Note also that some students taking this class might not have taken an undergrad algorithms class (for example if they were math/physics majors)

What does one teach in such a class ? There are a number of conflicting goals here:

Advertising: attract new students to work with you via the class. Obviously, you'd like to test them out first a little (this is not the only way of doing this)

Pedagogy: there's lots of things that someone might need even if they don't do algorithms research: topics like approximations and randomization are not usually well covered in an intro algorithms class

Retaining mindshare: you don't want to teach graduate algorithms as a CLRS class because you'll bore all the people who took such a class. On the other hand, you zoom too far ahead, and you lose the people who come to CS from different backgrounds. And the Lord knows we're desperate nowadays for people to come to CS from different backgrounds (more on this in a later post).

This is obviously not a new problem, and there are numerous models for how to teach a graduate algorithms course, all with their own pros and cons. Some of the extreme points that define the spectrum of solutions are:

Teach to the top: dive straight into advanced topics; anyone who needs to catch up does so on their own time. This is probably the most appealing solution from a lecturer perspective, because it covers more interesting topics. It's also most likely to excite the few students who care. Be prepared for bumpy student evals, especially from disgruntled folks who just need a grade so that they can forget all about algorithms for the rest of their Ph.D

Teach to the bottom: Do a CLRS-redux, with maybe a few advanced topics thrown in at the end (NP-hardness might count as advanced). You'll bore the good ones, but you'll bring the entire class along.

A 'split-the-difference' solution of covering basic material quickly, and then jumping ahead into advanced topics, is also a possibility. Like most solutions of this kind though, it runs the risk of satisfying nobody while trying to satisfy everyone.

All of this elides the most important point though: what constitutes advanced material ? I've seen graduate students struggle with NP-hardness, but this is covered in almost any undergraduate algorithms class (or should). What about general technique classes like approximation algorithms and randomization, both of which merit classes in their own right and can only be superficially covered in any algorithms class (the same is true for geometry) ? Advanced data structures can span an entire course, and some of them are quite entertaining. Topics like FFTs, number-theoretic algorithms, (string) pattern matching, and flows also seem to fit into the purview of advanced topics, though they don't cohere as well with each other.

It seems like an advanced algorithms class (or the advanced portion of a graduate algorithms class) is a strange and ill-defined beast, defined at the mercy of the instructor (i.e ME !). This problem occurs in computational geometry as well; to use graph-theoretic parlance, the tree of topics fans out very quickly after a short path from the root, rather than having a long and well-defined trunk.